Acrylic structural adhesive compositions are well-known articles of commerce which are extensively used commercially for bonding metal and plastic materials. The load-bearing and stress-relieving properties of structural adhesives, as well as their bond strength, which can exceed the strength of the materials which are being bonded, make these adhesives attractive alternatives to or partial replacements for mechanical methods, such as riveting or spot welding, of joining materials, especially where it is preferable to distribute load stresses over larger areas rather than to concentrate such stresses at a few points. Their use can reduce or eliminate costly finishing operations necessitated by mechanical joining methods, present a more pleasing exterior and at least reduce the possibility of corrosion of assemblies containing one or more metal components. Additionally, they can be used to bond a diversity of metals without extensive surface preparation.
Acrylic structural adhesives are extensively used for providing structural strength-imparting bonds to joined metal and or polymer materials. Acrylic structural adhesives are useful for bonding of metal parts in place of welding or mechanical fastening techniques. The structural requirements include high bond strength and good failure mode. A typical method to measure bond strength is the lap shear, high speed impact, and T-peel tests. One prevalent use for acrylic structural adhesives is in forming hem flanges in automotive body panels and doors. Exemplary conventional acrylic structural adhesives and methods for using acrylic structural adhesives are disclosed in the following U.S. patents: U.S. Pat. No. 6,180,199 entitled Beaded Adhesive And Hem Flanged Part Made Therefrom; U.S. Pat. No. 6,074,506 entitled Method Of Bonding Using Non-Compressible Beads; U.S. Pat. No. 5,932,638 entitled Free, Radical Polymerizable Compositions Including Para-Halogenated Aniline Derivatives; U.S. Pat. No. 5,783,298 entitled Adhesive Mixture With Non-Compressible Beads Therein U.S. Pat. No. 5,710,235 entitled Olefinic And Urethane-Terminated Ester Polyalkadiene; U.S. Pat. No. 5,641,834 entitled Modified Polyalkadiene-Containing Compositions; and U.S. Pat. No. 5,632,413 entitled Adhesive Bonding Apparatus And Method Using Non-Compressible Beads. Conventional acrylic structural adhesives typically comprise a mixture of one or more olefinic reactive monomers such as methyl methacrylate and methacrylic acid, toughener(s) and redox initiator system. The toughener(s), which may or may not be reactive, or polymerizable with the reactive monomers. Reactive polymers such as unsaturated polyesters and acrylourethane prepolymers may be used to graft onto or crosslink the initiated monomers during polymerization. In addition, fully formulated acrylic structural adhesives typically contain other additives for improving adhesion to substrate materials, environmental resistance, impact strength, flexibility, heat resistance, and the like. Epoxy resins impart improved heat resistance.
Unfortunately, with many prior art structural adhesives there is a trade-off between flexibility/toughness and strength. In order to increase flexibility and toughness rubbery polymers are added to the adhesive, however the addition of these rubbery polymers negatively impacts the modulus and ultimately the strength of the adhesive. It would therefore be desirable to provide a structural adhesive formulation with increased flexibility and toughness without sacrificing strength.
It is to these needs that the present invention is directed.